1. Field of the Invention
The invention relates generally to progressing cavity well pumps and in particular to separating the gas from the crude oil before pumping the oil up the well.
2. Description of the Related Art
When an oil well is initially completed, the downhole pressure may be sufficient to force the well fluid up the well tubing string to the surface. The downhole pressure in some wells decreases, and some form of artificial lift is required to get the well fluid to the surface. One form of artificial lift is suspending a centrifugal electric submersible pump (ESP) downhole in the tubing string. The ESP provides the extra lift necessary for the well fluid to reach the surface. An ESP has a large number of stages, each stage having an impellor and a diffuser. In gassy wells, or wells which produce gas along with oil, there is a tendency for the gas to enter the pump along with the well fluid. Gas in the pump decreases the volume of oil transported to the surface, which decreases the overall efficiency of the pump and reduces oil production. A gas separator may be mounted between the pump and motor to reduce gas entering into the pump. The gas separator rotates at the same speed as the pump and motor.
A progressive cavity pump is another type of well pump. A progressing cavity pump has a helical metal rotor that rotates inside a helical elastomeric stator. The liquid being pumped acts as a lubricator between the helical rotor and the stationary stator. If gas enters the pump, the gas may prevent the liquid from continuously lubricating the rotor and stator surfaces while flowing through the pump. The stator deteriorates quicker when there is not a thin layer of liquid on their surfaces acting as a lubricator. Quicker deterioration of the stator causes less time between maintenance and repairs of the pump.
Gas separators have not been used in conjunction with progressing cavity pumps, which operate at slower speeds than centrifugal pumps. Furthermore, the shaft in a rotary separator has a concentric or substantially circular path around the centerline of the shaft, while the rotor of a progressing cavity pump has an eccentric or elliptical path around the centerline of the rotor.
The downhole pump assembly in this invention has a progressing cavity downhole pump that is suspended by tubing in a well. The progressing cavity pump is a positive displacement pump. A cavity of liquid is forcibly pushed through the pump when a helical-shaped rotor rotates inside of the stator. A motor drives the rotor of the pump with a drive shaft. However the drive shaft from the motor typically rotates at a speed that is too fast for the rotor of the pump. A gear assembly between the motor and the pump transmits the rotations from the drive shaft to the pump rotor at a slower, operational speed of the pump.
A separator located below the pump separates the gas from liquids in the well fluid. The separator may have a helical inducer and a series of vanes rotated by a separator shaft inside of the separator housing, which in turn is driven by the motor. Alternatively, the separator may have a vortex chamber instead of vanes after the helical inducer. One end of the separator shaft is connected to the rotor of the pump. The separator shaft travels in a concentric or substantially circular path around the centerline of the shaft, while the rotor of the pump travels in an eccentric or elliptical path around the centerline of the rotor. A flexible shaft connects the shaft of the separator to the rotor of the pump. The flexible shaft compensates for different paths of the rotor and the separator shaft.
An annular passageway is located in the area between the flexible shaft and a shroud or housing that encloses the flexible shaft. The annular passageway is in fluid communication with the liquid outlet from the separator and the liquid inlets of the pump. In the first embodiment, the separator is also located above the gear reduction unit. Therefore, in this embodiment, the vanes and helical inducer of the separator rotate at the same speed as the rotor of the pump.
After suspending the pump assembly in the well, power is supplied to the motor to rotate the separator shaft and the pump rotor. The gear reduction unit located below the separator decreases the rotational speeds of the separator shaft and the pump rotor from that of the drive shaft from the motor. Well fluids enter the separator through separator inlets at the lower portion of the separator. The well fluid flows into an optional rotating helical inducer, and delivers the fluids into the separator vanes. The rotating vanes use centrifugal forces to push the heavier liquids in the well fluid to the outermost portion of the separator while the lighter gases remain in the innermost portions of the separator.
The liquids on the outer portion of separator exit the vanes to a passage on the outer surface of a crossover lip. The gases exit the vanes to the inner surface of the crossover lip. The crossover communicates the separated gases to gas outlets on the exterior surface on the upper portion of the separator. The gases exit the separator and rise to the surface under normal gas-lift properties. The passageway on the outside of the crossover lip communicates the separated liquids to the separator outlets on the upper portion of the separator, above the gas outlets. The separator liquid outlets communicate with the annulus surrounding the flexible shaft inside of the housing. The annulus communicates the liquids the to inlets of the pump.
The liquids enter the progressing cavity pump into a cavity between the rotor and the stator. The cavity travels up the pump as the rotor rotates inside the stator. Most of the fluid travels with the cavity and exits out of the pump outlets on the upper portion of the pump into the tubing with an increased liquid pressure to lift the liquids to the surface. A thin layer of liquid typically remains on the surfaces of the rotor and the stator when the cavity carrying liquid passes through the pump. The thin layer of liquid acts as a lubricant between the rotor and the stator. The liquid continues to lubricate the rotor and stator surfaces during operation. Therefore, the stator does not deteriorate due to lack of lubrication.
In another embodiment, the gear reduction unit is located between the separator and the pump. In this embodiment, the shaft of the separator rotates at the same speed as the drive shaft from the motor, while the rotor of the pump still rotates at the slower pump speed. The shroud surrounding the flexible shaft between the pump and the separator also extends down around the gear reduction unit to a point below the pump liquid outlets. Liquid communicates from the pump outlets into an annular passage between the shroud and the gear reduction unit to the annulus between the shroud and the flexible shaft to the pump inlets. This embodiment is good for situations in which the separator needs to operate at a faster speed in order to separate the gas from the liquids in the well fluid.
In the third embodiment, a motor on the surface at the upper end of the well drives the pump and separator. The drive shaft from the motor has a drive member extending down the well to the rotor of the pump. The separator is connected to the pump by a flexible shaft enclosed in a housing, as in the first embodiment. The separator is also driven by the motor located on the surface. The separator shaft is rotating at the same speed as the rotor of the pump.
In all three of these embodiments, gas in the well fluid is separated from the liquid before the liquids enter the pump. These embodiments increase the amount of time between repairs of the rotor and stator of the pump because the pump is continuously lubricated.